1. Field of the Invention
The present invention relates to an optical wavelength filter and, more particularly, to an optical wavelength filter in the form of arrayed waveguide grating (AWG) for use in an optical coupler/splitter of optical waveguide type which separates input light having different wavelengths multiplexed into the wavelengths of light to output the separated wavelengths of light.
2. Description of the Background Art
In recent years, techniques using silicon (Si) as a waveguide material have begun to attract attention. Silicon has the feature that it is higher in refractive index than silicon oxide (SiO2) and polymeric materials and, therefore, such high-refractive-index materials are used in the cores of optical circuit devices of optical waveguide type. Use of those materials promotes miniaturization of optical circuit devices of optical waveguide type and mass-production of equipment employing such optical circuit devices. It is expected that those optical circuit devices will find various applications.
One example of application is an optical coupler for multiplexing different wavelengths of optical signal onto a single optical waveguide channel. Another example is an optical splitter for separating different wavelengths of optical signal multiplexed on a single optical waveguide from each other. The optical couplers and splitters are very important devices in a large-capacity optical communication system, known as wavelength division multiplexing (WDM), of multiplexing a large bundle of wavelengths over a single optical fiber for optical transmission. Furthermore, to an optical coupler or splitter that combines or splits different wavelengths of optical signal, preferably applicable are optical circuit devices of optical waveguide type, which does not require the optical axes thereof to be aligned.
The optical coupler/splitter may also be referred to as an optical wavelength filter, because of its nature of selectively transmitting certain wavelengths of light. Examples of optical wavelength filters of optical waveguide type acting as an optical coupler/splitter are of using a Mach-Zehnder interferometer, a directional coupler, or diffraction grating. In particular, as an optical wavelength filter of optical waveguide type applicable to separating plural signals of different wavelengths multiplexed on a light beam, an arrayed waveguide grating (AWG) having arrayed waveguides will be found.
Thus, optical circuit devices including an optical wavelength filter having the cores of its optical waveguides made of a high-refractive index material such as silicon can be miniaturized and mass-produced. An optical waveguide having its core made of silicon may be referred to as a silicon waveguide.
Silicon has the feature that its refractive index is significantly dependent upon temperature. Therefore, an optical wavelength filter having a silicon waveguide involves the problem that the wavelength characteristics vary with temperature. In order to overcome the difficulty in temperature dependency, various methods have been discussed. One specific example of solution is disclosed in J. Teng, et al., “Athermal Silicon-on-insulator ring resonators by overlaying a polymer cladding on narrowed waveguides”, Optics Express, Vol. 17, No. 17, pp. 14627-14633, Aug. 17, 2009. Another example is disclosed in M. Uenuma, et al., “Temperature-independent silicon waveguide optical filter”, Optics Letters, Vol. 34, No. 5, pp. 599-601, Mar. 1, 2009.
J. Teng, et al., discloses a solution of using a polymer clad in an optical wavelength filter having a silicon waveguide. This solution is the simplest measure in eliminating the temperature dependency of the silicon waveguide. However, polymeric materials involve intrinsic problems that they are inferior in durability and reliability to inorganic materials such as silicon.
M. Uenuma, et al., discloses a method of designing a Mach-Zehnder interferometer having a silicon waveguide having its optical waveguide structure not depending on temperature. This method is superior to J. Teng, et al., in that the former uses no polymeric material. M. Uenuma, et al., however, suffers from the problem that the method is not practical because of its nature of small or marrow allowance against dimensional errors introduced when fabricated and of restricted applications.
So far as optical wavelength filters having an optical waveguide having its core made of a quartz-based material are concerned, various solutions for removing the temperature dependency have been heretofore developed, as disclosed in U.S. Pat. No. 6,377,723 B1 to Saito et al., Japanese Patent Laid-Open Publication Nos. 2000-035523 and 2001-083339, and U.S. Pat. No. 6,542,685 B1 to Yoneda.
However, silicon is larger in temperature dependency of refractive index than quartz-based materials. Therefore, it would have been difficult to remove the temperature dependency if solutions for the quartz-based materials were applied to optical wavelength filters having a silicon waveguide.